A. Field of the Invention
This invention relates generally to machines that are used to prepare a physiological solution from a batch of chemicals, such as dialysis machines, including peritoneal and hemodialysis machines. More particularly, the invention relates to the process by which the solution is prepared and the conductivity of the solution is adjusted. The invention is particularly useful in conjunction with a machine that prepares and stores an entire batch of solution, such as a batch dialysate preparation system for a dialysis machine, as opposed to machines based on so-called "proportioning" systems, in which the solution is prepared continuously during the treatment session.
B. Statement of Related Art
Dialysis, including hemodialysis and peritoneal dialysis, is a treatment for patients that suffer from inadequate kidney function. In hemodialysis, blood is pumped from the patient's body through an extracorporeal artificial kidney circuit to a dialyzer. Blood-borne toxins and excess water are filtered out of the blood through a semipermeable dialyzer membrane into an electrolyte (dialysate) solution. In peritoneal dialysis, the patient infuses a quantity of dialysate into the peritoneal cavity, and the peritoneal membrane acts as the semipermeable membrane. After a dwell period, the dialysate fluid is drained and a fresh supply of peritoneal dialysate is added to the peritoneal cavity.
The dialysate solution used in hemodialysis and peritoneal dialysis machines has typically been prepared using a proportioning technique, wherein a concentrated dialysate solution is mixed with water to form a dialysate solution with the aid of a proportioning pump. To control the dialysate conductivity in a proportioning system, the proportioning pump is precisely regulated, or alternatively, the rate of supply of the dialysate powder and water to a mixing vessel is regulated. A representative dialysis machine that uses precise control of proportioning pumps is described in the Peterson et al. patent, U.S. Pat. No. 5,247,434.
In a batch system, unlike a proportioning system, an entire batch of dialysate (e.g., approximately 50 liters) is prepared shortly before the dialysis session and stored in a dialysate solution tank. See the patents to Kenley et al., U.S. Pat. No. 5,591,344, and Twardowski, U.S. Pat. No. 5,336,165 for descriptions of representative batch systems. Batch dialysate solution is typically prepared from a powdered sodium bicarbonate formulation and a liquid acid concentrate that are added to the dialysate solution tank and mixed to water to form an approximately 50 liter batch. The dialysate powder and liquid acid formulations may be stored in individual containers during shipping, and are opened at the time of preparation of the solution and their contents added to the tank.
Since the total volume of the dialysate tank and associated dialysate fluid circuits is reasonably constant and capable of being determined experimentally, one method of insuring that the concentration and conductivity of the resulting dialysate solution is accurate is to precisely control the quantity and formulation of the dialysate concentrates that are used to make up each batch of dialysate. This method would work if it were possible to also precisely control the total fluid volume of the dialysate fluid circuit including the dialysate tank.
In practice, the above method has disadvantages. First, precise control of the quantity and formulation of the dialysate concentrates in their individual vessels is difficult and expensive. Additionally, the total fluid volume of the dialysate circuit and tank can vary due to manufacturing tolerances and changes in components over the life cycle of the dialysis machine. If the tank and associated dialysate fluid circuit are subject to heat disinfection cycles, this can cause a slight expansion of the components resulting in an increase in system volume. It has been determined that even small fluctuations in total system volume can cause conductivity measurements that are out of a desired range, unless the quantity of the bicarbonate and liquid acid concentrates added during the preparation of the batch of dialysate are precisely controlled.
The present inventors have further appreciated that the process of preparing a batch of dialysate solution has its own difficulties, including the instability of the solution during the process of mixing the solution, the possibility of loss of carbon dioxide gas from the bicarbonate solution and resulting precipitation of calcium carbonate in the dialysate tank, and a substantial amount time being required to thoroughly mix a batch quantity of solution automatically in a compact machine suitable for use in environments outside of traditional dialysis clinic settings. The present invention describes methods of preparing the batch dialysate solution that overcome these and other difficulties.